Vibrational Spectra and Structure of Small Ring Molecules. XVI. Vibrational Analysis and Ring-Puckering Vibration of 1-Pyrazoline

Abstract

The infrared spectrum of gaseous 1‐pyrazoline has been measured from 33 to 4000 cm⁻¹. The infrared and Raman spectra of the corresponding liquid sample have also been recorded. A vibrational assignment of the fundamentals is proposed which is consistent with the $C_s$ equilibrium configuration. The assignment is based on the gas‐phase band contours, the depolarization values, and group frequency correlations. A series of 13 pronounced $Q$ branches was observed in the far‐infrared spectral region, and they have been attributed to the strongly anharmonic ring‐puckering vibration. Although the far‐infrared frequencies could be reproduced by two different potential functions, a double minimum potential of the form ${\text{V\hspace{0.17em}=\hspace{0.17em}23.40(z}}^4{\text{\hspace{0.17em}−\hspace{0.17em}4.40z}}^2)$ where $z$ is the reduced ring‐puckering coordinate provides the most satisfactory interpretation of all the spectral data. The height of the inversion barrier is calculated to be 113 cm⁻¹, and the equilibrium angle between the two dihedral planes of the puckered ring is 19.7° ± 1.0°. The results are compared with those obtained on cyclopentene.